Magnetic proximity sensing has been used for many years. There are many different magnetic proximity sensor concepts in use today including, static threshold, transient, electromagnetic, and solid-state. The list of prior art for magnetic proximity sensing applications is very long and includes security systems, automation, instrumentation, etc. These include a number of different approaches for tamperproof applications, primarily in high security systems. A sensing device such as a reed switch, other mechanical magnetic switch, such as the Magnasphere®, Hall Effect sensor, magneto-resistive sensor or other solid-state device is used for sensing the strength of the magnetic field or the resulting magnetic flux flow.
Most proximity sensors require two devices, one attached to each of the two elements (physical entities) whose relative proximity to each other is sensed. These devices typically comprise a permanent magnet opposite a reed switch, as is commonly used in security systems. These devices are vulnerable to external magnetic field that can defeat their integrity of maintaining security. Other methods of tampering include removing both the magnet and the sensor of the protected door, widow, etc., while maintaining the proximity and orientation between them.